Radio communication spectrum is a valuable natural resource. With the rapid development of wireless communication technology, the problem of poor spectrum resources is becoming increasingly serious, in order to relieve the current situation of constrained spectrum resources, relevant departments and agencies monitor and study the wireless communication spectrum, finding that some frequency bands (such as television frequency bands) are not used during most time or in most areas, while simultaneous competition on some frequency bands among multi-user & multi-system occurs. That is to say, some systems with a very large traffic load do not have enough spectrum resources, while other systems with a small traffic load occupy too many resources.
The concept of Cognitive Radio is just generated in this background, and its basic idea is: under the premise of causing no interference to incumbent systems, a cognitive system can dynamically select a white space frequency band to conduct communications by monitoring the change of the current radio environment.
Protection of the incumbent systems against interferences is a basic premise of the cognitive system's opportunistic usage of the white space band of the incumbent systems, and such protection includes essentially two aspects:
(1) The cognitive system accurately determines the frequency bands of which incumbent systems are available white space frequency bands (the introduction of the cognitive system on these frequency bands does not affect normal operation of the incumbent systems).
(2) When the occupied frequency bands are no longer available, the cognitive system needs to concede these frequency bands to the incumbent systems timely.
There are primarily three methods for protecting the incumbent systems:
Method I. Database Method
In the current cognitive radio standard studies, the database method acquires more approval. Taking the FCC (Federal Communications Commission) standard as an example, when the incumbent system is a broadcast television system, the incumbent system's information database needs to provide information, including:
A. Geographic coordinates of base stations, e.g., the longitudes and latitudes specified in NAD83 (North American Datum in 1983);
B. Channel numbers (i.e., the used frequency bands) of TV (television);
C. Effective Radiated Power;
D. HAAT (Height Above Average Terrain) of transmission antennas;
E. Horizontal directional patterns of transmission antennas (only for directional antennas);
F. Inclination angles and clockwise azimuths from the due north direction of the antennas; and
G. Call signs of radio stations.
The database may be placed in a separate server or directly placed in a base station of a cognitive system and timely updated according to the change of the radio environment, and a cognitive base station may determine which frequency bands of the incumbent systems are available using various information in the database in combination with the physical location of the base station itself (which is acquired by a GPS or Beidou navigation satellite system module) and other information.
Method II. Cognitive Method
In the cognitive method, the base station of the cognitive system detects the signal strength of the incumbent system on a certain frequency band by a sensing algorithm, when the signal strength is greater than a threshold value, then the distance between this base station and the incumbent system using this frequency band is considered to be too close, and introduction of the cognitive system on this frequency band will affect the normal operation of the incumbent system, thus this frequency band is thus unavailable.
Method III. Method of Combining Database Method and Cognitive Method
The method of combining the database method and the cognitive method combines the advantages of both methods: for the system whose white space frequency bands have a certain regularity, the database method may operate better; but when the cognitive base station cannot connect to the database or the incumbent system information in the database is incomplete, the cognitive method may play a flexible and effective supplemental role.
In either method, acquisition of the isolation distance between the cognitive base station and the incumbent system and comparison of the isolation distance with a tolerable threshold value is critical.
As shown in FIG. 1, it is a schematic diagram of interference of the wireless cognitive technology in the prior art, where a BS tower is a transmission tower of an incumbent system, a BS receiver is a fixed receiving device of the incumbent system, and cognitive radio is a transmission (receiving) device of the cognitive system. It can be seen that during a co-channel or adjacent-channel operation, the transmission (receiving) device of the cognitive system needs to be spaced from the receiving device of the incumbent system by a certain distance to ensure the normal operation of the incumbent system, i.e., Link 1 in the figure cannot be too strong.
In the implementation process of the present invention, the inventor has found that at least the following problems exist in the prior art:
All the above three methods only take into account the interference of the cognitive system to the incumbent system without considering the interference of the incumbent system to the cognitive system (i.e., the link shown in dashed lines in FIG. 1), and thus the normal operation of the cognitive system cannot be guaranteed.
As can be seen from this, the existing methods for determining available white space spectrums of the incumbent system only take into account the interference of the cognitive system to the incumbent system without considering the interference of the incumbent system to the cognitive system, and thus, the “white space spectrums” detected by such methods are not necessarily truly available.
For example, when the incumbent system is a digital broadcasting system and the cognitive system is a TD-LTE (Time Division Long Term Evolution) system, the interference of transmission tower of the digital broadcasting system to the base station of the TD-LTE system is very large, the distance isolation requirement between the two systems will be much stricter than the distance isolation requirement for only protecting the digital broadcasting system, and the TD-LTE system is completely inoperable on the “white space spectrums” determined by the presently available methods.